Systems and methods for RFID positioning

ABSTRACT

Systems and methods are disclosed for radiolocation using backscatter RFID tags and a special-purpose reader that produces a stepped-frequency continuous wave (SFCW) radio frequency (RF) interrogation signal comprising N carrier frequencies. A backscattered interrogation signal from a backscatter RFID tag is down-converted using at least a portion of the generated SFCW RF interrogation signal. Received signal phases (RSP) corresponding to the N carrier frequencies are determined. Received signal strength (RSS) may be determined to improve performance. A distance between the RFID reader and the backscatter RFID tag may be estimated based on at least a summation of differences between RSPs corresponding to adjacent carrier frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 63/148,870, filed on 12 Feb. 2021, the contents of which areincorporated herein by reference in their entirety as if fully set forthbelow.

FIELD OF THE DISCLOSURE

The various embodiments of the present disclosure relate generally toimproved systems and methods for determining locations of radiofrequency identification (RFID) tags, and more particularly to improvedtechniques for improving ranging and/or positioning accuracy.

BACKGROUND

Radiolocation determination in realistic environments can be inaccuratedue to factors such as multipath echoes and fading. High-accuracypositioning is typically performed with optical sensors that have alimited range, long setup time, and environmental limitations,particularly when used outdoors in sunlight.

Radio frequency identification (RFID) systems typically involve the useof small, low-cost, electronic RFID tags that store informationincluding identification information. Backscatter RFID tags include anantenna that can receive an interrogation signal from a reader, andcircuitry that can modulate and reflect/backscatter the modulated signalso that the reader device can wirelessly interrogate a tag and receivesuch identification information. The tags may be placed on equipment,vehicles, pallets, or people, for example, and the correspondingidentification of the tags may be determined by the reader.

It is often desirable to obtain the location of items having attachedRFID tags. The typical RFID reader can obtain identification informationfor tags located within the range of the reader, but obtaining highlyaccurate distance or location information is still a challenge.

In some cases, RFID tags and/or RFID readers may be moving orstationary. In many situations, it is desirable to determine the preciselocation of a tag and/or reader.

Current motion capture technology involves bulky sensor boxes and hasranges of only a few meters. Traditional location trackers usingbackscattering RFID tags can provide certain advantages such as smallersizes, reduced complexity, and low power requirements; however, theyhave limited ranges, low ranging precision, and typically must be placedin the line-of-sight (LoS) of an RFID reader.

For stable wireless communication and localization withInternet-of-things (IoT) devices, an accurate propagation channel modelis needed. Traditional channel modeling relies on complicated slidingcorrelator systems or bulky lab equipment such as Vector NetworkAnalyzers (VNAs), synthesized sweepers, etc. In practice, localoscillator synchronization is difficult to achieve, and often a cable isconnected between source and measurement nodes to share a commonfrequency reference, which greatly limits the range of measurements.There is a need for a versatile fine-scale localization technology thatoperates in realistic environments. Such technology could enablenumerous commercial and scientific sensing applications.

BRIEF SUMMARY

The disclosed technology relates to systems and methods forradiolocation using backscatter RFID tags and a special purpose RFIDreader that interrogates the backscatter RFID tags with astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal. A distance between the reader and the tag may bedetermined by processing the backscattered signal. Certainimplementations also enable sensing structures via processing multipathcomponents of the backscattered signal.

According to an exemplary implementation of the disclosed technology, amethod is provided for radiolocation. The method includes generating, bya signal generator of a radio frequency identification (RFID) reader, astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal comprising N carrier frequencies; splitting off atleast a portion of the generated SFCW RF interrogation signal androuting it to a local down converter; transmitting at least a portion ofthe generated SFCW RF interrogation signal by a transmitting (Tx)antenna of the RFID reader; receiving by a receiving (Rx) antenna of theRFID reader and in response to the transmitting, a backscattered signalfrom a backscatter RFID tag; down-converting the received backscatteredsignal using at least a portion of the generated SFCW RF interrogationsignal; determining, from the down-converted signal, received signalphases (RSP) corresponding to the N carrier frequencies; and estimatinga distance between the RFID reader and the backscatter RFID tag based ona summation of differences between RSPs corresponding to adjacentcarrier frequencies.

In accordance with an exemplary implementation of the disclosedtechnology, a radio frequency identification (RFID) radiolocation systemis provided that includes: a signal generator configured to output astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal comprising N carrier frequencies; a transmitting(Tx) antenna; a receiving (Rx) antenna configured to receive abackscattered signal from one or more backscatter RFID tags; adown-converter configured to output a down-converted signal comprisingan in-phase (I) and a quadrature-phase (Q) output; a splitter incommunication with the signal generator, the Tx antenna, and thedown-converter, wherein the splitter is configured send a first portionof the SFCW RF interrogation signal to the Tx antenna and is furtherconfigured to send a second portion of the SFCW RF interrogation signalto the down-converter; a software-defined radio configured to digitizeand filter the down-converted signal from the down-converter. The systemincludes one or more processors in communication with thesoftware-defined radio, the one or more processors are configured todetermine, from the down-converted signal, received signal phases (RSP)corresponding to the N carrier frequencies; estimate a distance betweenthe RFID radiolocation system and the backscatter RFID tag based on asummation of differences between RSPs corresponding to adjacent carrierfrequencies, and output the estimate of the distance.

According to an exemplary implementation of the disclosed technology, anon-transitory computer-readable storage medium is provided. The mediumis configured for storing instructions for use with one or moreprocessors in communication with: a signal generator; a software-definedradio; and memory. The instructions are configured to cause the one ormore processors to perform a method comprising: generating, by thesignal generator of a radio frequency identification (RFID) reader, astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal comprising N carrier frequencies; splitting off atleast a portion of the generated SFCW RF interrogation signal androuting it to a local down converter; transmitting at least a portion ofthe generated SFCW RF interrogation signal by a transmitting (Tx)antenna of the RFID reader; receiving by a receiving (Rx) antenna of theRFID reader and in response to the transmitting, a backscattered signalfrom a backscatter RFID tag; down-converting the received backscatteredsignal using at least a portion of the generated SFCW RF interrogationsignal; determining, from the down-converted signal, received signalphases (RSP) corresponding to the N carrier frequencies; and estimatinga distance between the RFID reader and the backscatter RFID tag based ona summation of differences between RSPs corresponding to adjacentcarrier frequencies.

These and other aspects of the present disclosure are described in theDetailed Description below and the accompanying drawings. Other aspectsand features of embodiments will become apparent to those of ordinaryskill in the art upon reviewing the following description of specific,exemplary embodiments in concert with the drawings. While features ofthe present disclosure may be discussed relative to certain embodimentsand figures, all embodiments of the present disclosure can include oneor more of the features discussed herein. Further, while one or moreembodiments may be discussed as having certain advantageous features,one or more of such features may also be used with the variousembodiments discussed herein. Similarly, while exemplary embodiments maybe discussed below as device, system, or method embodiments, it is to beunderstood that such exemplary embodiments can be implemented in variousdevices, systems, and methods of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of specific embodiments of thedisclosure will be better understood when read in conjunction with theappended drawings. To illustrate the disclosure, specific embodimentsare shown in the drawings. It should be understood, however, that thedisclosure is not limited to the precise arrangements andinstrumentalities of the embodiments shown in the drawings.

FIG. 1 is a block diagram representation of an RFID reader in accordancewith certain exemplary implementations of the disclosed technology.

FIG. 2 illustrates an example arrangement of an RFID readerinterrogating an RFID tag and receiving a modulated signal from the RFIDtag, in accordance with certain exemplary implementations of thedisclosed technology.

FIG. 3 illustrates an example GNU radio implementation, according tocertain aspects of the disclosed technology.

FIG. 4 is an example logic flow diagram for data processing frequencyhopping, in accordance with certain exemplary implementations of thedisclosed technology.

FIG. 5 a illustrates an example transmitted stepped-frequency continuouswave (SFCW) in the frequency domain (top) and time-domain (bottom), inaccordance with certain exemplary implementations of the disclosedtechnology.

FIG. 5 b illustrates an example of received backscatter signal amplitude(top) and phase (bottom), in accordance with certain exemplaryimplementations of the disclosed technology.

FIG. 5 c illustrates a time-domain example of post-processed signalsderived from the backscatter signal, which can enable ranging andchannel sounding, in accordance with certain exemplary implementationsof the disclosed technology.

FIG. 6 illustrates a top view of an experimental arrangement formeasuring distance, in accordance with certain exemplary implementationsof the disclosed technology.

FIG. 7 a is a graph of delay profile results calculated using the IDFTof normalized received signal experimental data.

FIG. 7 b is a graph of delay profile results calculated using the IDFTof un-normalized received signal experimental data. In accordance withcertain exemplary implementations of the disclosed technology, theadditional RSS information of un-normalized received signals canincrease the detection accuracy of multipath components.

FIG. 8 shows measured RSS values of slow hopping mode in the frequencydomain, where dips in the values were caused by pedestrians and vehiclesmomentarily obstructing the line-of-sight measurements.

FIG. 9 a shows error estimation using slow hopping (dwell time=10 ms).

FIG. 9 b shows error estimation using fast hopping (dwell time=2 ms).

FIG. 10 a shows mean error estimation using slow hopping (dwell time=10ms).

FIG. 10 b shows mean error estimation using fast hopping (dwell time=2ms).

FIG. 11 illustrates a quantum tunneling tag, in accordance with certainexemplary implementations of the disclosed technology.

FIG. 12 a illustrates a trilateration approach in which the location isestimated by intersecting two circles that identify the distance of anobject from a target.

FIG. 12 b illustrates another trilateration approach in which thelocation can be estimated.

FIG. 13 illustrates a motion-tagging use case in which a plurality oftags and readers may be employed, in accordance with certainimplementations of the disclosed technology.

FIG. 14 is a flow diagram of a method, according to an exemplaryimplementation of the disclosed technology.

DETAILED DESCRIPTION

The disclosed technology includes systems and methods that utilizeunique properties of a radio frequency identification (RFID) backscatterlink that can be used to sound the channel with respect to an absolutetime scale, thereby enabling accurate ranging and/or location estimates.The disclosed technology further enables the identification and removalof multipath effects from a location estimate.

By measuring the received signal phase and/or the received signalstrength, the disclosed technology can accurately estimate the distancebetween a special-purpose RFID reader and an RFID tag. Certain exemplaryimplementations of the disclosed technology may determine channelcharacteristics by calculating the Inverse Discrete Fourier Transform(IDFT) of the received signal in the frequency domain. Certain exemplaryimplementations of the disclosed technology enable measuring a delayprofile of the backscatter channel to estimate channel characteristicsin various multipath environments, which is not achieved by any otherRFID techniques.

When compared with the state-of-the-art RFID localization systems, theproposed technology gives both higher communication ranges and higheraccuracy. Furthermore, most traditional positioning techniques tend tohave higher percentage positioning errors when the distances increase;the proposed technology, instead, has a lower percentage positioningerror despite the longer ranges.

Since the use of microwave frequencies (e.g.: the 5.8 GHz ISM band) isstill uncommon in the RFID industry, the disclosed technology mayutilize a custom measurement system to make a real-time frequencyhopping coherent reader that extracts the received signal phase (RSP)from a received signal, processes the data, and presents the calculatedposition on a graphical user interface (GUI) in real-time.

Certain exemplary implementations of the disclosed technology include abackscatter channel sounder that utilizes a frequency hopping readerthat hops between multiple frequency channels at 5.8 GHz ISM band, whichcan provide channel modeling for backscatter communication systemswithout complicated wired setup and synchronization. Ultra-precise,submeter-scale position at long distances in a real-world environmentcan also be achieved using the disclosed technology, which can utilizethe RSP of the backscattered signals from an RFID tag. In certainimplementations, a tunneling tag (for example, one that utilizes anEsaki tunneling diode) may be utilized to improve amplification ofbackscattered signals while preserving the linear relationship betweenthe reader-to-tag distance and RSP.

To facilitate an understanding of the principles and features of thepresent disclosure, various illustrative embodiments are explainedbelow. The components, steps, and materials described hereinafter asmaking up various elements of the embodiments disclosed herein areintended to be illustrative and not restrictive. Many suitablecomponents, steps, and materials that would perform the same or similarfunctions as the components, steps, and materials described herein areintended to be embraced within the scope of the disclosure. Such othercomponents, steps, and materials not described herein can include butare not limited to, similar components or steps that are developed afterthe embodiments disclosed herein.

FIG. 1 is a block diagram representation of an RFID reader system 100 inaccordance with certain exemplary implementations of the disclosedtechnology. The RFID reader system 100 can be considered a “channelsounder” that utilizes a signal generator 102 to produce astepped-frequency continuous-wave (SFCW) signal 103 (see inset box inFIG. 1 ). In certain exemplary implementations, the signal generator 102hops between multiple frequencies in the 5.8 GHz ISM band, however, thedisclosed technology is not limited to this frequency range as otherfrequency bands in the 860-960 MHz and/or 2.4 GHz frequency range, forexample may be utilized. One or more of the dwell time, frequencydifferences between hops or steps (Δf=f_(n+1)−f_(n)), the number offrequencies N, and/or sequence/order of frequencies may each beselectively varied. In an experimental setup, a frequency hopping rangeof 5.725 GHz to 5.825 GHz was utilized, with a frequency bandwidthchosen to be 101 MHz with 101 evenly distributed channels (Δf=1 MHz);however, the disclosed technology may utilize other ranges, steps,bandwidth, etc.

In accordance with certain exemplary implementations of the disclosedtechnology, the SFCW signal 103 may be split using splitter 104 (e.g.,RF power divider) with a first portion 105 of the (split) SFCW signal103 routed to a transmit (Tx) antenna 106 for output 107 to interrogatean RFID tag 108. A receive (Rx) antenna 110 may receive a backscattersignal 109 from the (remote) RFID tag 108, and this received backscattersignal 109 may be amplified with an amplifier 112. A second portion 113of the (split) SFCW signal 103 may be routed internally to adown-converter 114 that may mix with the (received and amplified)backscatter signal 109 to produce in-phase (I) and quadrature-phase (Q)signals for input to a universal software radio peripheral (USRP)receiver 116 for sampling and processing the I/Q signals. The output ofthe USRP receiver 116 may be output to a computer 118 (having one ormore processors) for additional processing, display, etc. In certainexemplary implementations, the computer 118 may also be utilized tocontrol the signal generator 102.

FIG. 2 illustrates another example system 200 and arrangement having anRFID reader 202 configured for interrogating an RFID tag 206 with aninterrogation signal 204 and receiving a modulated backscatter signal208 from the RFID tag 206, in accordance with certain exemplaryimplementations of the disclosed technology. Some or all of thecomponents of this example system 200 may correspond to like componentsof the RFID reader system 100 discussed above with respect to FIG. 1 .

An inherent advantage of the disclosed technology is the near-perfectphase coherence of the carrier since the same oscillator 203 (or signalgenerator 102 in FIG. 1 ) is used in both the transmitting and thereceiving chains. Therefore, the received complex baseband signal afterdemodulation can be expressed as:

$\begin{matrix}{{{\overset{\sim}{S}\left( f_{c} \right)} = {{A\left( f_{c} \right)}{\exp\left\lbrack {- {j\left( {{\varphi_{ps}\left( f_{c} \right)} + {\varphi_{m}\left( f_{c} \right)} + {\varphi_{0}\left( f_{c} \right)}} \right)}} \right\rbrack}}},} & (1)\end{matrix}$where A(f_(c)) is the magnitude of the received signal at the carrierfrequency f_(c),

$\begin{matrix}{{{\varphi_{ps}\left( f_{c} \right)} = \frac{4\pi f_{c}d}{c}},} & (2)\end{matrix}$is the phase shift due to the propagated distance d, φ₀(f_(c)) is thephase offset caused by the propagation within hardware (e.g. cables,antennas, tag modulation, and other reader components), and φ_(m)(f_(c))is the phase offset caused by the multipath channel.

With the received signal phase (RSP)-based method disclosed herein, theestimated distance between a reader and a tag can be calculated usingthe received signal phase:

$\begin{matrix}{{\hat{d} = {\frac{\lambda_{e}}{4\pi N}{\sum\limits_{n = 1}^{N - 1}{❘{{\varphi_{n + 1}\left( f_{n + 1} \right)} - {\varphi_{n}\left( f_{n} \right)}}❘}}}},} & (3)\end{matrix}$

with φ_(n) and φ_(n+1) being the measured phases of the received signalsobtained by the reader at carrier frequencies f_(n) and f_(n+1),respectively; N is the number of the frequency channels; and

$\lambda_{e} = \frac{c}{\Delta f}$the equivalent wavelength obtained when a uniform frequency step,Δf=f_(n+1)−f_(n) is used. In accordance with certain exemplaryimplementations of the disclosed technology, a maximum detection range{circumflex over (d)}_(max) of the RSP-based method may be determined bythe minimum frequency step of the reader:

$\begin{matrix}{{\hat{d}}_{\max} = {\frac{c}{2\Delta f} = {\frac{\lambda_{e}}{2}.}}} & (4)\end{matrix}$

FIG. 3 is a block diagram of an example implementation 300 that mayutilize a GNU radio and frequency division multiple access (FDMA),according to certain aspects of the disclosed technology, to collectmultiple data in real-time from different tags and to enableradiolocation by trilateration. In this example implementation 300,transmit blocks 302 may receive commands from a frequency hoppingcontrol block 304. Receive blocks 306 may output sampled signals to aplurality of custom filters 308 (one of each tag), and the RSP may beprocessed by corresponding data processing blocks 310. The output of thedata processing blocks 310 may be fed to a location engine 312 todetermine. In accordance with certain implementations of the disclosedtechnology, the FDMA may be used in reverse configuration (three fixedtags in this example) to detect the real-time position of a movingreader. The receiving section 306 may collect three differentfrequencies backscattered by each tag and may process them throughGoertzel filters 308, for example, with pre-set center frequencies,which may act as a single point FFT filter with very narrow bandwidth.The filters 308 may extract the RSP from each tag and, when enough dataare collected, the position of the moving reader may be measured anddisplayed on the GUI in real-time. In certain exemplary implementations,copies of the received raw data output may be saved locally forpost-processing. In accordance with certain exemplary implementations ofthe disclosed technology, the transmitting section may implement thefrequency hopping controller 304 that lets the reader interrogate thetags at different frequencies.

FIG. 4 is an example logic flow diagram 400 for data processingfrequency hopping implementation in real-time using a GNU radio, inaccordance with certain exemplary implementations of the disclosedtechnology. The reader may transmit an unmodulated continuous wave (forexample, at 5.785 GHz) until it receives back enough data to process agood estimation of the RSP, which may be set by or correspond to thedwell time (as discussed with respect to FIG. 1 ). In certain exemplaryimplementations, a command may be sent to set the transmit frequency tothe next adjacent frequency channel, for example, by increasing thetransmit frequency by 1 MHz. Once all channels are swept, thereader-to-tag distance may be calculated, the transmit frequency may bereset (for example, to 5.725 GHz) and the positioning measurement mayrestart.

FIGS. 5 a-5 c illustrate the channel sounding procedure in accordancewith certain implementations of the disclosed technology.

FIG. 5 a illustrates an example transmitted stepped-frequency continuouswave (SFCW) in the frequency domain (top) and time-domain (bottom), inaccordance with certain exemplary implementations of the disclosedtechnology.

FIG. 5 b illustrates an example of received backscatter signal amplitude(top) also known as the received signal strength (RSS) and RSP (phase)at each frequency channel (from f₁ to f_(N)) in accordance with certainexemplary implementations of the disclosed technology.

The amplitude and the phase of the received backscattered signals can beexpressed as:

$\begin{matrix}{{{P_{RSS}(f)} = \frac{{I_{rx}(f)}^{2} + {Q_{rx}(f)}^{2}}{Z_{0}}},} & (5)\end{matrix}$ $\begin{matrix}{\left. {{\varphi_{RSP}(f)} = {{\arctan\left( \frac{Q_{rx}(f)}{I_{rx}(f)} \right)} \in \left\lbrack {0,2\pi} \right.}} \right),} & (6)\end{matrix}$where Z₀ is the impedance of the RF circuits, f represents the hoppingcarrier frequencies of the reader, and I_(rx) and Q_(rx) are thereceived signals in the baseband for each carrier. Since thedown-converted and the transmitted signals share the same signalgenerator, the channel sounding reader can perform near-perfect coherentphase detection. Moreover, to increase the signal-to-noise ratio (SNR),a GNU Radio built-in Geortzel filter may be used, as discussed abovewith respect to FIG. 3 . Although the transmitted signal may be uniformat each channel, the RSS usually changes for each frequency due tomultipath, different frequency responses of the system, temporarilyblocked line-of-sight (LoS), etc.

In a multipath free environment, the RSP offset φ_(ps) caused by the LoSround-trip propagation can be expressed as

$\begin{matrix}{{\varphi_{ps} = {- \frac{4\pi d}{\lambda}}},} & (7)\end{matrix}$where λ is the wavelength at the carrier frequency f. Therefore, the RSPat different carrier frequencies varies due to the differentwavelengths. In certain implementations, the RSPs, like the RSSs, mayalso be affected by the environment, therefore the differential RSPbetween adjacent channels is usually not a constant as equation (7)suggests.

In accordance with certain exemplary implementations of the disclosedtechnology, the RSPs may be utilized to calculate the distance{circumflex over (d)} between the reader and the tag, as discussed abovewith reference to equations (3) and (4) above.

Wireless engineers often model the multipath channels as a collection ofdiscrete multipath components constituting a power delay profile, p(τ),as a function of delay (τ) as:

$\begin{matrix}{{{p(\tau)} = {\sum\limits_{i}{p_{i}{\delta\left( {\tau - \tau_{i}} \right)}}}},} & (8)\end{matrix}$with p_(i) being the backscattered signal received with a time delayτ_(i). In the frequency domain, the normalized and un-normalized complexreceived signal can be expressed as:

$\begin{matrix}{{{C(n)}_{norm} = {\sum\limits_{n = 1}^{N}{\exp\left( {{- j}\phi_{n}} \right)}}},} & (9)\end{matrix}$ $\begin{matrix}{{{C(n)}_{{un} - {norm}} = {\sum\limits_{n = 1}^{N}{R_{n}{\exp\left( {{- j}\phi_{n}} \right)}}}},} & (10)\end{matrix}$where R_(n) and ϕ_(n) are respectively the RSS and RSP at the n-thfrequency channel with N being the total number of channels determinedby the frequency span and step frequency.

Given the signal in the frequency domain, the discrete power delayprofile in time delay domain, p(τ_(k)), can be derived using the InverseDiscrete Fourier Transform (IDFT) of the complex received signal C(n) inthe frequency domain

$\begin{matrix}{{{p\left( \tau_{k} \right)} = {{\frac{1}{N}{\sum_{n = 1}^{N}{{C(n)}{\exp\left( \frac{{- j}2\pi n\tau_{k}}{N} \right)}}}} = {\frac{1}{N}{\sum_{n = 1}^{N}{R_{n}{\exp\left( {{- j}\phi_{n}} \right)}}}}}},} & (11)\end{matrix}$where τ_(k) represents the discrete two-way travel time delay inequation (8).

FIG. 5 c illustrates a time-domain example of post-processed signalsderived from the backscatter signal, which can enable ranging andchannel sounding, in accordance with certain exemplary implementationsof the disclosed technology. The delay profile after IDFT may berepresented by a summation of Kronecker delta functions with variousamplitudes from τ₁=1 to τ_(N)=N as:

$\begin{matrix}{{p\left( \tau_{k} \right)} = {\sum\limits_{k = 1}^{N}{p_{k}{{\delta\left( {\tau - \tau_{k}} \right)}.}}}} & (12)\end{matrix}$

Both normalized and un-normalized received signals can be used tocalculate the delay profile. The former only requires RSP while thelatter needs a good estimation of both RSS and RSP. However, quantumtunneling RFID tags have non-uniform gain depending on the impingingpower level and its frequency response. Thus, a delay profile generatedusing both normalized and un-normalized received signals may be used. Toestimate the distance traveled by the backscattered signal, thediscrete-time delay τ_(k) in equation (11) can be converted to one-waytravel distance using:

$\begin{matrix}{{d = {\frac{c}{2B}\tau_{k}}},} & (13)\end{matrix}$where c is the speed of light and B is the bandwidth. The distancebetween the reader and tag can then be determined by the travel distanceof the first arrival component of the delay profile. The resolution ofthe discrete one-way travel distance and the ambiguous distance aredetermined by both the bandwidth and the number of channels,respectively:

$\begin{matrix}{{d_{res} = \frac{c}{2B}},{and}} & (14)\end{matrix}$ $\begin{matrix}{d_{\max} = {\frac{c}{2B}{N.}}} & (15)\end{matrix}$

Zero-padding is a common technique used to increase the resolution offrequency resolution of the Discrete Fourier Transform (DFT). Byappending the actual signal in the frequency domain to a zero vectorwith a length M, the resolution of the IDFT in the time delay domain canalso be improved, resulting in a better distance estimate resolution.The improved distance resolution can be expressed as:

$\begin{matrix}{d_{res} = {\frac{c}{2B}{\frac{N}{N + M}.}}} & (16)\end{matrix}$

FIG. 6 illustrates a top view of an experimental setup 600 for measuringthe distance from an RFID reader 601 to a corresponding tunneling tagsequentially placed and measured in 20-meter increments 602, 604, 606,608, 610. The measurements were made to test the channel sounder and toinvestigate the impact of multipath and interference from multiplebuildings in the vicinity. In this experimental setup 600, the RFIDreader 601 was used to transmit unmodulated continuous wave signals inthe 5.8 GHz ISM band, to receive the modulated signals backscattered byone tunneling tag at a time, and to down-convert the received signalsinto baseband I and Q components. An E-shaped patch antenna was used onthe receiving front-end of the reader 601 while an omnidirectionalmonopole antenna was used for the tunneling tag. As shown in FIG. 6 ,the reader 301 was placed nearby Building 1 614 with the antennas facingBuilding 3 616 at about 114 m away.

The tunneling tag was moved 20 in meter increments 602, 604, 606, 608,610 on a straight path between the two buildings 614 616 starting at adistance of 20 meters from the reader 601 and ending to 100 meters fromthe reader 601. An external wall 612 was located about 114 meters fromthe reader 601. Along the path, trees and bushes were always near thereader 601. A Wi-Fi router 618 located on the exterior wall of Building2 620 was left powered on despite the possible interference it couldhave introduced to the system. To estimate the multipath delay spectrumand reader-to-tag distance, the reader 601 hopped from 5.725 GHz to5.875 GHz with a step frequency of 200 kHz for each position 602, 604,606, 608, 610 of the tag. To study the impact of dwell time on theranging accuracy, the reader 601 was configured to dwell on each channelfor 200 ms. In post-processing, different lengths of data were extractedto study the effects of various dwelling times. In particular, twodifferent dwell times were investigated by using part of the digitizedreceived signal of each channel, the slow hopping (10 ms) and the fasthopping (2 ms).

FIG. 7 a is a graph of delay profile results calculated using the IDFTof normalized received signal data using the experimental setup 600shown in FIG. 6 and equation (9) above (IDFT norm) for a tunneling tagthat was moved 20 in meter increments 602, 604, 606, 608, 610 on astraight path between the two buildings starting at a distance of 20meters from the reader 601 and ending to 100 meters from the reader 601,as shown in FIG. 6 . A reflection from the external wall 612 can be seenin this graph.

FIG. 7 b is a graph of delay profile results calculated using the IDFTof un-normalized received signal experimental data using theexperimental setup 600 shown in FIG. 6 and equation (10) above (IDFTun-norm). In accordance with certain exemplary implementations of thedisclosed technology, the additional RSS information of un-normalizedreceived signals can increase the detection accuracy of multipathcomponents, as can be seen in the wall reflection 612 shown in FIG. 7 b, as compared to the same wall reflection 612 shown in FIG. 7 a.

The slow hopping configuration (dwell time of 10 ms) was used tocalculate the average RSP for each frequency channel. Both normalized(FIG. 7 a ) and un-normalized delays (FIG. 7 b ) provide enough data toestimate the LoS component of the backscatter signals. However, withun-normalized delays (FIG. 7 b ), the reflections 612 from the wall(about 115 meters away from the reader) can be better distinguished fromthe noise floor at each set of measurements.

FIG. 8 is a chart 800 showing measured RSS (received signal strength)values of the tunneling tag (measured using the setup 600 of FIG. 6 )using the slow hopping mode in the frequency domain, where dips in thevalues were caused by pedestrians and vehicles momentarily obstructingthe line-of-sight measurements. Due to the increase of reflective gainof the tunneling tag at lower impinging power levels, predictions on tagposition become more difficult at longer distances, hence, theestimation error increases when based on RSS only. Note that themeasurement results (depicted in FIGS. 7 a, 7 b , and 8) includeinterference caused by the campus WiFi 618 router (as shown in FIG. 6 )resulting in a strong interference between 5.755 GHz and 5.575 GHz inmost measurements, particularly when the tag is placed at 40 meters (604FIG. 6 ). In this situation, the tunneling tag is saturated due to thehigh combined impinging power from the reader and the stronginterference, resulting in a reduced gain. As shown in FIG. 8 , the RSSwith the tag at 40 meters is approximately at the same level as the tagat 60 meters and even at 80 meters at some frequencies, which causes anestimated delay profile at 40 meters and also shows a higher noise levelcompared to other distances.

Certain implementations of the channel sounding method disclosed hereincan also be used to estimate the position of a tag by processing at thereceived LoS components of the backscattered signal, while thereader-to-tag distance may be estimated using equation (3) with only thedifferential RSP. The measured reader-to-tag distance includes the wavetravel distance in cables and circuits, which is a constant and can becalibrated by using the first measurement at 20 m as a reference.

FIG. 9 a shows error estimation using a slow hopping reader (dwelltime=10 ms) using RSP from equation (3) and two different methods IDFTnorm from equation (9) and IDFT un-norm from equation (10). With the RSPbased method, the mean, the RMS, and the 90th percentile errors of0.86%, 1.28%, and 2.50%, respectively, are observed. The IDFT methodreduces the 90th percentile error to 1.34% (by 46.4%), while thenormalization does not significantly change the distance estimationaccuracy. Both IDFT methods with or without normalization have a meanerror of less than 0.47% and an RMS error of less than 0.65%.

The distance estimation accuracy also depends on many other factors,such as dwell time, number of frequency channels, the existence ofinterference, and thermal noise. Dwell time determines how fast thesystem can estimate the location of the tag. Although it may bebeneficial to have faster prediction times, taking average RSP of moredata when using a slower hopping reader can reduce the impact of thethermal noise and give a more accurate estimation of the RSP when theSNR is low.

FIG. 9 b shows error estimation using fast hopping reader (dwell time=2ms) using RSP from equation (3) and two different methods: IDFT normfrom equation (9) and IDFT un-norm from equation (10). Compared to theRSP-based method, the IDFT methods perform significantly better in termsof both mean error (reduced from 2.87% to less than 1.01%) and 90thpercentile error (reduced from 6.67% to less than 2.44%).

According to certain exemplary implementations, the data shows that theIDFT methods can lead to better distance estimation when using a fasthopping reader. In addition, the extra RSS information in theun-normalized IDFT method may not provide higher accuracy than thenormalized IDFT method that only requires RSP.

In accordance with certain exemplary implementations of the disclosedtechnology, the signal-to-noise ratio (SNR) and signal-to-interferenceratio (SIR) may also be factors for accurate distance estimation usingthe systems and methods disclosed herein. For example, in theexperimental setup 600 discussed above with reference to FIG. 6 , thecampus Wi-Fi 618 operating in the same frequency band as the reader 601can suppress the gain of the tunneling tag and may cause distanceestimation offsets.

FIG. 10 a shows mean error estimation at each distance using slowhopping (dwell time=10 ms). When the tag is placed at 40 m, the tagreceives the highest level of interference, causing a dramatic increasein the estimation error. This is also reflected in the higher 90thpercentile error for the RSP-based method in FIG. 9 a.

FIG. 10 b shows mean error estimation at each distance using fasthopping (dwell time=2 ms). When using the fast hopping mode, theaccuracy of both RSP- and IDFT-based methods may decrease at mostdistances. The RSP-based method may suffer from higher estimation errordue to low SNR at longer distances (at 80 m and 100 m), while IDFTmethods remain highly accurate (<1.26%) at such distances. In general,the data indicates that the IDFT methods may outperform the RSP-basedmethod by a factor of 2 to 3.

Quantum Tunneling Tags

FIG. 11 illustrates a quantum tunneling tag 1100 that may be utilizedwith the disclosed technology. As discussed in “Tunneling RFID Tags forLong-Range and Low-Power Microwave Applications,” Amato et al., IEEEJournal of Radio Frequency Identification. PP. 1-1.10.1109/JRFID.2018.2852498, which is incorporated herein by reference,as if presented in full, traditional passive RFID systems have powerconstraints that can limit RFID tag communication to short ranges.However, certain exemplary implementations of the disclosed technologymay overcome some of these limitations by employing quantum tunnelingtags.

The experimental results as discussed above with respect to FIGS. 6-10 bwere obtained using a quantum tunneling tag. Quantum tunneling tags areunique in that they have non-uniform gain depending on the impingingpower level and its frequency response. Certain exemplaryimplementations of the disclosed technology may use a tunnelingreflector that can act as a switching load for the tunneling tag.Certain exemplary implementations of the disclosed technology may takeadvantage of the natural negative differential resistance of a tunneldiode 1102 in the quantum tunneling tag 1100. While passive tags canhave a maximum sensitivity of −22.1 dBm that corresponds to a maximumforward link range of 6.6 m at 5.8 GHz, tunneling tags, by beingsensitive to RF input signals as low as −85 dBm, can reach a theoreticalforward link of 9 km.

Exemplary Use Case 1 Localization with RSP-Based Ranging

Certain implementations of the disclosed technology may be utilized fora radiolocation system that can be extended to two and three-dimensionpositioning systems by using multiple and appropriate frequency hoppingreaders and tags. By measuring the distances of a tag from three (orfour) readers at known locations, its 2D (or 3D) position can beextrapolated using a trilateration approach.

Trilateration is widely used in many well-known real-time positioningsystems. Once the distance between a reader and a tag is computedthrough equation (3), the target device (such as a tag or reader) may belocated on a circumference of a radius. To estimate the actual 2Dlocation of the tag, the intersection of at least two circumferences isneeded, however, the position is still not precise since the tag can beon either of the two intersection points or, in case of estimationerrors, anywhere within the area of intersection. One way to eliminatethe ambiguous point is by adding a third reader. Directional or Van Attaantennas may be utilized to further increase the position accuracy.

FIG. 12 a illustrates a trilateration approach according to thedisclosed technology, in which the location of a target device (tag) maybe estimated by intersecting two circles surrounding readers thatidentify the distance from the corresponding reader to a target device,and where a third reader may be used to disambiguate the location of thetarget device. FIG. 12 a depicts an example configuration, where a taglocation is unknown (and possibly moving) and three or more readers(base stations) are fixed in location.

FIG. 12 b illustrates another trilateration approach in which a locationof a reader (target device) is unknown or moving and three (or more)fixed tags (base stations) act as anchor points that enable the locationof the reader to be estimated using trilateration methods describedabove. Tests were conducted using this configuration to estimate thereader-to-tag distances for each reader location with three fixed tags,and a tag-to-reader distance ranging from about 6 to about 9 meters. Toachieve accurate positioning, calibration was performed to eliminatemeasurement offset caused by internal wave traveling within the RFcables, transmission lines, matching networks, etc. The same reader wasused for all measurements, enabling the offset to be calibrated out byreference measurements. However, when different tags are used, each tagmay perform differently, may need separate impedance matching and/orseparate calibrations due to manufacturing imperfections. By placing thereader at a known location, the calculated distance {circumflex over(d)} can be used to determine the value of the offset d_(o) byd_(o)=d−{circumflex over (d)}, where d is true reader-to-tag distance.Nevertheless, estimation errors caused by time-varying factors (e.g.:tag movements due to the windy weather, changing multipath caused bypeople moving around the reader, etc) also exist, which may not becalibrated out by reference measurements.

A first set of measurements at the first known reader location were usedto calculate the offset d_(o) for each tag based on 120 and 40 datapoints for 1D and 2D positioning, respectively. A mean error and an RMSerror of 0.11 m and 0.14 m were observed for 1D estimations. While, in2D, the calculated positions provide a mean and an RMS error of 0.17 mand 0.20 m, respectively. Considering the size of the reader's cart(about 1 m by 0.5 m) and the placement error, the achieved positioningerror is smaller than the size of the target. Moreover, the estimatedposition of the reader is both accurate and precise. Note that thedistance estimation error remains the same while the reader-to-tagdistance increases. Thus, the percentage error is expected to be lowerat longer ranges. Similar accuracy improvements are expected for usingthis system in a larger area covered by more tags or readers. Althoughonly the reverse positioning system has been shown in this set ofexperiments, the same performance is expected for the configuration inwhich multiple fixed readers are used for localizing one or more movingtags.

Exemplary Use Case 2 Motion Tracking and Capture

Motion tracking and capture have recently been used as a basis forcomputer animation in many applications, including but not limited totelevision, video games, sports, and education. In the traditionalsystems, a performer wears visibly reflective markers near each joint,the scene is typically captured by one or more video cameras, and theoutput from the cameras may be used to compute by the positions orangles between the markers so that the relative position of the markerin space may be computed.

FIG. 13 illustrates a motion-capture use case in which the disclosedtechnology may be utilized to determine positions of a plurality of tags1302 in real-time. As shown, M low-profile backscatter tags 1305 may beattached to the target (a ballerina in this case) and N readers may beemployed to extract/disambiguate the location of each of the tags (forexample, as discussed above with reference to FIGS. 12 a and/or 12 b).The disclosed technology may provide both higher spatial and temporalresolution compared with traditional systems.

In certain exemplary implementations, each backscatter tag 1302 may havea unique ID which may help eliminate marker swapping and may providemuch cleaner data than other technologies. In certain exemplaryimplementations, the use of the SFCW RF interrogation signal and thebackscatter RFID tags may enable motion capture outdoors in directsunlight and may provide better results for lower operational costs.

FIG. 14 is a flow diagram of a method 1400 of radiolocation, accordingto an exemplary implementation of the disclosed technology. In block1402, the method 1400 includes generating, by a signal generator of aradio frequency identification (RFID) reader, a stepped-frequencycontinuous wave (SFCW) radio frequency (RF) interrogation signalcomprising N carrier frequencies. In block 1404, the method 1400includes splitting off at least a portion of the generated SFCW RFinterrogation signal and routing it to a local down converter. In block1406, the method 1400 includes transmitting at least a portion of thegenerated SFCW RF interrogation signal by a transmitting (Tx) antenna ofthe RFID reader. In block 1408, the method 1400 includes receiving by areceiving (Rx) antenna of the RFID reader and in response to thetransmitting, a backscattered signal from a backscatter RFID tag. Inblock 1410, the method 1400 includes down-converting the receivedbackscattered signal using at least a portion of the generated SFCW RFinterrogation signal. In block 1412, the method 1400 includesdetermining, from the down-converted signal, received signal phases(RSP) corresponding to the N carrier frequencies. In block 1414, themethod 1400 includes estimating a distance between the RFID reader andthe backscatter RFID tag based on a summation of differences betweenRSPs corresponding to adjacent carrier frequencies.

In certain exemplary implementations, the backscatter RFID tags includeone or more quantum tunneling tags (QTT).

Certain exemplary implementations of the disclosed technology caninclude one or more of determining a received signal strength (RSS),determining a received signal phase (RSP) from the down-convertedsignal, determining a normalized complex received signal from thedown-converted signal, determining an un-normalized complex receivedsignal from the down-converted signal, determining a delay profile usingboth the normalized complex received signal and un-normalized complexreceived signal, and/or estimating an improved distance measurement RFIDreader and the backscatter RFID tag based on the delay profile.

In certain exemplary implementations, the backscattered signal receivedfrom the one or more backscatter RFID tags can include a version of theSFCW RF interrogation signal that is modulated and backscattered by theone or more backscatter RFID tags.

In accordance with certain exemplary implementations of the disclosedtechnology, the down-converting may be performed by a localdown-converter of the RFID reader. In certain exemplary implementations,the local down-converter may be configured to output an in-phase (I) andquadrature (Q) output corresponding to the backscattered signal.

In certain exemplary implementations, the SFCW RF interrogation signalmay be characterized by a sequence of stepped frequencies in a 5.8 GHzindustrial, scientific, and medical (ISM) band. In certain exemplaryimplementations, the stepped frequencies may be characterized by a dwelltime.

Certain exemplary implementations of the disclosed technology mayutilize two or more RFID readers to triangulate the location of thebackscatter RFID tag. Certain exemplary implementations of the disclosedtechnology may utilize three RFID readers to triangulate anddisambiguate a location of the backscatter RFID tag.

Certain exemplary implementations of the disclosed technology mayutilize two or more RFID readers to trilaterate the location of thebackscatter RFID tag. Certain exemplary implementations of the disclosedtechnology may utilize three RFID readers to trilaterate anddisambiguate a location of the backscatter RFID tag.

Certain exemplary implementations of the disclosed technology mayinclude receiving by the (Rx) antenna of the RFID reader and in responseto the transmitting, backscattered signals from a plurality ofbackscatter RFID tags and estimating a distance between the RFID readerand the plurality of backscatter RFID tags.

Certain exemplary implementations of the disclosed technology caninclude sensing one or more structures between the RFID reader and thebackscatter RFID tag based on delayed or multipath components of thebackscattered signal.

It is to be understood that the embodiments and claims disclosed hereinare not limited in their application to the details of construction andarrangement of the components set forth in the description andillustrated in the drawings. Rather, the description and the drawingsprovide examples of the embodiments envisioned. The embodiments andclaims disclosed herein are further capable of other embodiments and ofbeing practiced and carried out in various ways. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting theclaims.

Accordingly, those skilled in the art will appreciate that theconception upon which the application and claims are based may bereadily utilized as a basis for the design of other structures, methods,and systems for carrying out the several purposes of the embodiments andclaims presented in this application. It is important, therefore, thatthe claims be regarded as including such equivalent constructions.

We claim:
 1. A method of radiolocation, comprising: generating, by asignal generator of a radio frequency identification (RFID) reader, astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal comprising N carrier frequencies; splitting off atleast a portion of the generated SFCW RF interrogation signal androuting the generated SFCW RF interrogation signal to a local downconverter; transmitting at least a portion of the generated SFCW RFinterrogation signal by a transmitting (Tx) antenna of the RFID reader;receiving by a receiving (Rx) antenna of the RFID reader and in responseto the transmitting, a backscattered signal from a backscatter RFID tag,wherein the backscatter RFID tags comprise one or more quantum tunnelingtags (QTT); down-converting the received backscattered signal using atleast a portion of the generated SFCW RF interrogation signal;determining, from the down-converted signal, received signal phases(RSP) corresponding to the N carrier frequencies and a received signalstrength (RSS); and estimating a distance between the RFID reader andthe backscatter RFID tag based on a summation of differences betweenRSPs corresponding to adjacent carrier frequencies; determining anormalized complex received signal from the down-converted signal;determining an un-normalized complex received signal from thedown-converted signal; determining a delay profile using both thenormalized complex received signal and un-normalized complex receivedsignal; and estimating an improved distance measurement RFID reader andthe backscatter RFID tag based on the delay profile and outputting theimproved estimate of the distance.
 2. The method of claim 1, wherein thebackscattered signal received from the one or more backscatter RFID tagscomprises a version of the SFCW RF interrogation signal that ismodulated and backscattered by the one or more backscatter RFID tags. 3.The method of claim 1, wherein the down-converting is performed by alocal down-converter of the RFID reader, and wherein the localdown-converter is configured to output an in-phase (I) and quadrature(Q) output corresponding to the backscattered signal.
 4. The method ofclaim 1, wherein the SFCW RF interrogation signal is characterized by asequence of stepped frequencies in a 5.8 GHz industrial, scientific, andmedical (ISM) band, and each of the stepped frequencies arecharacterized by a dwell time.
 5. The method of claim 1, furthercomprising utilizing three or more RFID readers to triangulate alocation of the backscatter RFID tag.
 6. The method of claim 1, furthercomprising receiving by the (Rx) antenna of the RFID reader and inresponse to the transmitting, backscattered signals from a plurality ofbackscatter RFID tags and estimating a distance between the RFID readerand the plurality of backscatter RFID tags.
 7. The method of claim 1,further comprising sensing one or more structures between the RFIDreader and the backscatter RFID tag based on delayed or multipathcomponents of the backscattered signal.
 8. A radio frequencyidentification (RFID) radiolocation system, comprising: a signalgenerator configured to output a stepped-frequency continuous wave(SFCW) radio frequency (RF) interrogation signal comprising N carrierfrequencies; a transmitting (Tx) antenna; a receiving (Rx) antennaconfigured to receive a backscattered signal from one or morebackscatter RFID tags, wherein the backscatter RFID tags comprise one ormore quantum tunneling tags (QTT); a down-converter configured to outputa down-converted signal comprising an in-phase (I) and aquadrature-phase (Q) output; a splitter in communication with the signalgenerator, the Tx antenna, and the down-converter, wherein the splitteris configured send a first portion of the SFCW RF interrogation signalto the Tx antenna and is further configured to send a second portion ofthe SFCW RF interrogation signal to the down-converter; asoftware-defined radio configured to digitize and filter thedown-converted signal from the down-converter; and one or moreprocessors in communication with the software-defined radio, the one ormore processors are configured to: determine, from the down-convertedsignal, received signal phases (RSP) corresponding to the N carrierfrequencies and a received signal strength (RSS); estimate a distancebetween the RFID radiolocation system and the backscatter RFID tag basedon a summation of differences between RSPs corresponding to adjacentcarrier frequencies; and output the estimate of the distance; determinea normalized complex received signal from the down-converted signal;determine an un-normalized complex received signal from thedown-converted signal; determine a delay profile using both thenormalized complex received signal and un-normalized complex receivedsignal; and estimate an improved distance measurement between the RFIDradiolocation system and the backscatter RFID tag based on the delayprofile, and output the improved estimate of the distance.
 9. The systemof claim 8, wherein the backscattered signal received from the one ormore backscatter RFID tags comprises a version of the SFCW RFinterrogation signal that is modulated and backscattered by the one ormore backscatter RFID tags.
 10. The system of claim 8, wherein the SFCWRF interrogation signal is characterized by a sequence of steppedfrequencies in a 5.8 GHz industrial, scientific, and medical (ISM) band,and each of the stepped frequencies are characterized by a dwell time.11. The system of claim 8, further comprising three or more of theradiolocation systems configured to triangulate a location of thebackscatter RFID tag.
 12. The system of claim 8, further comprising oneor more amplifiers.
 13. A non-transitory computer readable storagemedium storing instructions for use with one or more processors incommunication with: a signal generator; a software defined radio; andmemory; and wherein the instructions are configured to cause the one ormore processors to perform a method comprising: generating, by thesignal generator of a radio frequency identification (RFID) reader, astepped-frequency continuous wave (SFCW) radio frequency (RF)interrogation signal comprising N carrier frequencies; splitting off atleast a portion of the generated SFCW RF interrogation signal androuting the generated SFCW RF interrogation signal to a local downconverter; transmitting at least a portion of the generated SFCW RFinterrogation signal by a transmitting (Tx) antenna of the RFID reader;receiving by a receiving (Rx) antenna of the RFID reader and in responseto the transmitting, a backscattered signal from a backscatter RFID tag,wherein the backscatter RFID tags comprise one or more quantum tunnelingtags (QTT); down-converting the received backscattered signal using atleast a portion of the generated SFCW RF interrogation signal;determining, from the down-converted signal, received signal phases(RSP) corresponding to the N carrier frequencies and a received signalstrength (RSS); and estimating a distance between the RFID reader andthe backscatter RFID tag based on a summation of differences betweenRSPs corresponding to adjacent carrier frequencies; determining anormalized complex received signal from the down-converted signal;determining an un-normalized complex received signal from thedown-converted signal; determining a delay profile using both thenormalized complex received signal and un-normalized complex receivedsignal; and determining an improved distance measurement RFID reader andthe backscatter RFID tag based on the delay profile and outputting theimproved estimate of the distance.